1. Field of the Invention
This invention relates to an optical device such as a lens used in a bar-code reader or an image scanner for converging light from a light emitting element (a light source) to a recording surface and reading its reflected light at a light receiving part, or a lens for converging light from a light source such as a laser diode and inputting it to an optical fiber.
2. Description of the Related Art
FIG. 4 is an illustrative view for showing an optical device and FIG. 5 is also an illustrative view for showing an evaluating process of the lens to be used in an optical coupler device.
The optical device shown in FIG. 4 is comprised of a light source 1 composed of a laser diode, a lens 2 for focusing the light emitted from the light source 1 and an optical fiber 3 having an end surface 3a for receiving the focused light passed through the lens 2.
In this optical device, the light from the light source 1 is magnified and projected toward the end surface 3a of the fiber 3 at a predetermined magnification, although as the diameter of the beam projected against the end surface 3a of the fiber 3 approaches the diameter of the mode field of the fiber 3, the coupling efficiency of light with respect to the fiber 3 is increased.
For example, it is known in general that the laser diode to be used in optical communications may produce a laser beam with a half-angle of divergence .theta. of 30.degree. at its vertical and lateral widths, although it is assumed that the laser beam is focused by the lens 2 onto the end surface 3a of the fiber 3 having a diameter of about 10 .mu.m under a magnification of about 5 times. The coupling efficiency .eta. at the fiber 3 can be expressed as follows, EQU .eta.=.eta..sub.1 .times..eta..sub.2
where .eta..sub.1 is an incident coefficient of light of the light entering into an effective diameter of the lens 2, and .eta..sub.2 is a mode coupling coefficient between the focused laser beam and the fiber 3.
In this case, the value .eta..sub.1 is defined in response to the characteristic of the radiating angle of the laser beam and the numerical aperture NA of the effective diameter of the lens. The mode coupling coefficient .eta..sub.2 is defined as ##EQU1## where .omega..sub.f is a mode field radius of the fiber 3 and .omega..sub.1 is a beam waist radius of the laser beam focused on the end surface 3a of the fiber, the value .eta..sub.2 becoming a maximum when .omega..sub.1 .times..omega..sub.f. That is, when the mode waist radius of the fiber 3 and the beam waist radius at the end surface 3a of the fiber are coincident to each other, the optical coupling efficiency .eta. of the fiber 3 becomes a maximum.
However, in a practical optical device, it often occurs that the beam waist focused by the lens does not coincide with the mode waist of the fiber 3 due to a displacement of relative positions of the light source 1, the lens 2 and the fiber 3. In particular, in the case of a removable connector type optical device in which the light source 1 and the lens 2 are arranged on a connector, and the fiber 3 is located on a plug, the aforesaid displacement in relative position may occur every time the plug and connector are separated and reconnected. When a relative positional displacement occurs, because the mode coupling coefficient .eta..sub.2 is sensitively varied due to a displacement in a direction (a direction X) perpendicular to an optical axis L.sub.o between the optical axis L.sub.o of the laser beam and the fiber 3, even a slight displacement causes the coupling efficiency .eta. to be substantially varied.
A relation between the displacement amount Xo and the mode coupling coefficient .eta..sub.2 is defined as ##EQU2## where a relative displacement amount in a direction X between the optical axis Lo and the fiber 3 is defined as Xo. In view of the equation above, it is apparent that the relative displacement amount Xo may influence the mode coupling coefficient .eta..sub.2 significantly.
The field radius .omega..sub.f of the elements influencing the mode coupling coefficient .eta..sub.2 is defined by the fiber 3 and the beam waist diameter .omega..sub.1 is defined by a magnification of the lens 2. That is, in the case that the magnification of the lens 2 is increased, the beam waist diameter .omega..sub.1 is increased. In the case that the diameter .omega..sub.1 is increased, the mode coupling coefficient .eta..sub.2 is decreased, resulting in that the coupling coefficient .eta. is slightly deteriorated and a rate of variation of the mode coupling coefficient .eta..sub.2 with respect to the aforesaid displacement amount Xo can be reduced and, at the same time, influence of the displacement amount Xo generated in case of fixing or removing of the connector against the coupling efficiency .eta. can be decreased. Thereby, an allowable degree for the positional displacement Xo can be increased when the beam waist diameter W.sub.1 is increased.
However, upon increasing of the magnification of the lens 2, the slight variation of the relative position between the light source 1 and the lens 2 is amplified on the image forming side, thereby influencing image formation. That is, mere variation of the distance Y in the optical axis direction between the light source 1 and the lens 2 may cause substantial variation of the beam waist diameter .omega..sub.1 and also the mere slight variation of the relative position between the light source 1 and the lens 2 toward the vertical direction in respect to the optical axis Lo causes the displacement amount Xo to be widely varied at the image forming side. In the aforesaid removable type connector, the light source 1 and the lens 2 are arranged at the same connector side, so that at a first stage of manufacturing, the light source 1 and the lens 2 must be assembled with the relative position between the light source 1 and the lens 2 being kept at quite high accuracy, resulting in that the manufacturing operation is not only complicated, but also the fine positional displacement may occur between the light source 1 and the lens 2 due to the variation of a linear expansion coefficient at the supporting part of the connector and this may have a substantial influence at the image forming side.
In view of the foregoing, an object of the present invention aims at an optional enlargement of a surface aberration at the focusing position of the light passed through the lens 2 as a method in which the variation of the aforesaid displacement amount Xo does not substantially influence variations of the mode coupling. coefficient .eta..sub.2 without increasing the magnification of the lens 2. FIG. 6 illustrates schematically the light coupling state under this condition. In FIG. 6, Po indicates a variation of light converging rate of light at the beam waist when the surface aberration is minimized. In addition, Pm indicates a mode pattern of the fiber 3. In the case of the incident light shown at Po, it is apparent that the displacement amount Xo may substantially influence variations of the light coupling efficiency. In turn, the variation of the light collecting rate at the beam waist, when the surface aberration at the focusing position is enlarged, is indicated by P1. In this case, the image at the beam waist produces a certain non-focused image, and it is apparent that the relative displacement amount Xo of the fiber 3 provides less influence over the optical coupling efficiency.
Such a lens described above enables the optical surface 4 to be a non-spherical surface and the surface aberration at the focusing position can be increased under the design of the non-spherical surface shape. However, even in this case, the performance of the lens 2 must be maintained and the manufacturing of the lens and the evaluation of the lens parts must be carried out. However, the current evaluating method for determining the characteristics of the lens cannot be used to judge if the aberration of a lens is less than a predetermined aberration value or more than the predetermined value.
That is, as shown in FIG. 5, the general method for evaluating the performance of the lens in the prior art is performed such that the lens 2 is arranged between the interferometer 6 and the reference spherical surface 7, the interference between the incident light and the reflection light from the reference spherical surface is measured by the interferometer, and then the interference pattern is observed to confirm that the aberration is less than a predetermined value. In the evaluating method using the interferometer as described above, it is not possible to evaluate if the aberration of the lens at the focusing position is more than the predetermined value.
The present invention solves the aforesaid prior art problems and it is an object of the present invention to provide an optical coupling device in which a surface aberration at the incident position for the fiber is increased, and evaluation of the lens and the control of the lens can be carried out.